Convolvulus silvaticus Kit. (syn.: Calystegia silvatica (Kit.) Griseb., C. sylvestris (Waldst. et Kit. ex Willd.) Roem. et Schult., C. sepium var. sylvestris (Waldst. et Kit. ex Willd.) Willk., C. sepium subsp. silvatica (Kit.) Battand.) (Medit.) – A very rare but probably overlooked escape from cultivation. Discovered in abundance on the verge of a former clay-pit (now dump) in Roeselare in 2008 but obviously present since quite a long time. Furthermore seen on a heap of debris near a plant nursery in Dadizele in 2008 (apparently ephemeral). Also observed as a well-established plantation weed near a cemetery in Tielt in 2009. A mixed population with native Convolvulus sepium and intermediate forms was discovered in 2009 along a disused railway track near Eernegem. Furthermore seen in a fence along river Leie in Menen, also since 2009 (and subsequently confirmed). In 2011 new populations were discovered in Veurne (two localities) and Koksijde. Doubtlessly overlooked elsewhere. A previous, older Belgian record from Clermont-lez-Walcourt (1935) (Lawalrée 1956) turned out to belong to Convolvulus dubius (see above).
Convolvulus silvaticus is well-naturalised in large parts of Europe (and much commoner than C. dubius) and probably overlooked in Belgium (see also van der Meijden 2005, Holverda & al. 2009, Verloove 2011). It is a rather variable species in which two subspecies have been described (Brummitt 1996). To our knowledge Belgian plants belong to subsp. silvaticus which is very characteristic in having strongly saccate, very broad bracteoles (about twice as broad as long) and large corollas (55-90 mm long). A less typical form with less saccate, narrower bracteoles and smaller corollas (50-65 mm long) is much more reminiscent of native Convolvulus sepium. It was described as Calystegia silvatica subsp. disjuncta Brummitt and is confined to the western Mediterranean region. It is not impossible that the latter subspecies in fact belongs to the hybrid swarms of Convolvulus sepium and C. silvaticus.
Brummitt R.K. (1996) Two subspecies of Calystegia silvatica (Kit.) Griseb. (Convolvulaceae) in the Mediterranean region. Lagascalia 18(2): 338-340.
Brummitt R.K. (2002) Calystegia silvatica (Convolvulaceae) in western North America. Madroño 49(2): 130-131.
Holverda W., van Moorsel R. & Duistermaat L. (2009) Nieuwe vondsten van zeldzame planten in 2005, 2006 en ten dele 2007. Gorteria 34: 1-40.
Lawalrée A. (1956) Phanérogames adventices ou naturalisées en Belgique. Bull. Soc. Roy. Bot. Belg. 88: 25-28.
Van der Meijden R. (2005) Heukels’ Flora van Nederland (23e druk). Wolters-Noordhoff, Groningen: 685 p.
Verloove F. (2011) Weinig gekende Calystegia-taxa in België. Dumortiera (in press).